What should the dimension of the input be for text summarization?
Briefly:

I think what you are looking for is an RNN network (Either LSTM or GRU) with the many-to-many topology.

Explanation:

Clearly your input is the sentences (or to be more precise, the an embedding of your sentences, because you cannot feed the raw text to the network). then for each sentence you want to assign a value, which means for n inputs, you need n outputs. This is the many-to-many architecture.

Moreover, you might want to check the Bi-directional LSTM for your study. Not relevant to your question, but worth mentioning.

For more information, refer to this